1. Field of the Invention
The present invention relates to a process for the formation of resist patterns using a specific positive-working resist material and a specific developer suitable for the resist material. According to the pattern formation process of the present invention, fine resist patterns having an increased sensitivity and excellent resolution can be obtained, while avoiding a reduction of the layer thickness in an unexposed area of the resist coating and the formation of resist residues in an exposed area thereof. Therefore, the present process can be advantageously utilized in the production of semiconductor devices such as semiconductor integrated circuits, for example, Large Scale Integrated Circuits (LSIs) or Very Large Scale Integrated Circuits (VLSIs).
2. Description of the Related Art
Recently, the degree of integration of the semiconductor integrated circuits has been remarkably increased, and, for example, LSIs and VLSIs are now widely used as essential elements in a variety of electronic devices. LSIs and VLSIs, as is well-known in the art, can be produced, because it is now possible to obtain fine patterns of conductive circuit lines and electrodes. For example, it is possible to obtain fine patterns having a minimum width of less than 1 .mu.m, i.e., on a submicron order.
As an exposure source for the formation of fine resist patterns, ultraviolet (UV) rays was widely utilized, because of many advantages thereof, but since these rays have a relatively long wavelength, the resist patterns obtained using the UV rays as the exposure source were limited to those having a minimum pattern or line width of about 1.5 .mu.m.
To further reduce the possible minimum pattern width of the resulting resist patterns, it was found that ionizing radiations, such as electron beam (EB) or X-rays, could be used instead of UV rays as the exposure source. For example, the EB is sufficient to obtain fine resist patterns on the submicron order because it has a remarkably short wavelength of generally around 0.1 .ANG..
The resist process based on the EB exposure can be classified into two types, depending upon the surface configuration of the underlying material to be coated with the resist patterns. One resist process is used to apply resist patterns onto a flat surface of the underlying material, and another resist process is used to apply resist patterns onto an uneven or stepped surface of the underlying material. The former process is particularly suitable for the production of reticles and masking means, and the latter process is particularly suitable for the production of multilayered circuits. Note, the resist process of the present invention, as described hereinafter in detail, concerns the former process.
The inventors of this application recently discovered a novel positive-working electron beam resist. The EB resist, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 63-137227, published on Jun. 9, 1988, is characterized by consisting of a copolymer of e-methylstyrene and .alpha.-chloroacrylic ester of the formula: ##STR2## in which R represents an alkyl group such as methyl or haloalkyl group, and
l and m each are not equal to zero. PA1 X represents a halogen atom, and PA1 m and n each is larger than 0 and smaller than 100, and by developing the selectively exposed coating of the resist material under specific development conditions. PA1 applying a positive-working resist material to a substrate; the resist consisting of a copolymer of .alpha.-alkylstyrene and .alpha.-haloacrylic ester of the above-described formula (I); PA1 exposing the resulting coating of the resist material to a pattern of radiations; and PA1 developing the pattern-wise exposed resist with a developer which essentially consists of xylene for 10 to 20 minutes to remove the resist in an exposed area thereof. PA1 applying a positive-working resist material to a substrate; the resist consisting of a copolymer of .alpha.-alkylstyrene and .alpha.-haloacrylic ester of the above-described formula (I); PA1 exposing the resulting coating of the resist material to a pattern of radiations; and PA1 developing the pattern-wise exposed resist with a developer which essentially consists of at least one solvent selected from the group consisting of aromatic hydrocarbon solvents exclusive of xylene, and ketone solvents, to remove the resist in an exposed area thereof.
Surprisingly, the described EB resist showed a good adhesion to the underlying material, such as a silicon substrate and a good sensitivity, in addition to an excellent resolution and resistance to dry etching, when a coating thereof was selectively exposed to the electron beam and then developed with xylene, for example, for 3 to 5 minutes. A longer development time was avoided, because it causes a reduction of the layer thickness in an unexposed area of the resist coating while increasing an EB sensitivity of the resist. Note, An object of the invention of Japanese Kokai 63-137227 is to provide a novel EB resist having a higher sensitivity than that of poly(methyl methacrylate) (PMMA) and a higher resistance to dry etching than that of poly(butene sulfone) (PBS), and therefore it does not disclose the extension of the development time which causes additional drawbacks. Also note, in the EB resist of Japanese Kokai 63-137227, the excellent resistance thereof to dry etching relies upon the presence of a benzene ring of .alpha.-methyl-styrene, and the high sensitivity thereof relies upon the presence of .alpha.-chloro group in .alpha. -chloroacrylic ester.
As described above, the EB resist of Japanese Kokai 63-137227 shows a high sensitivity, but there is a need to more increase the sensitivity of the EB resist, to widen the scope of use thereof. Increasing the amount of EB irradiated on the resist causes an accumulation of the heat in the resist on a quartz applying to a reticle and thus induces a deformation of the resulting resist patterns and other defects. These defects were notably observed in the production of fine resist patterns on the submicron order during the EB lithographic process.